US20030168209A1

US20030168209A1 - Heat transfer tube with ribbed inner surface

Info

Publication number: US20030168209A1
Application number: US10/382,378
Authority: US
Inventors: Christoph Walther
Original assignee: Wieland Werke AG
Current assignee: Wieland Werke AG
Priority date: 2002-03-07
Filing date: 2003-03-06
Publication date: 2003-09-11
Also published as: EP1342971A2; DE10210016A1; DE10210016B4; DE10210016B9; EP1342971A3; CN1444003A; JP2003262486A

Abstract

A heat transfer tube (1) having a ribbed inner surface which is divided into at least two zones (Z₁, Z₂, . . . , Z_n) in circumferential direction, which zones are classified into at least two zone classes (K₁, K₂, . . . , K_j+1, . . . , K_m). Ribs (1) having a rib height h₁extend at a helix angle α₁with respect to the longitudinal direction of the tube in the zones of at least one zone class (K₁, K₂, . . . , K_j). Ribs (2) having a rib height h₂extend at a helix angle α₂in the zones of at least one further zone class (K_j+1, K_j+2, . . . , K_m), which ribs (2) are intersected by ribs (3) having a rib height h₃extending at a helix angle α₃(α₃≠α₂). Preferably are h₂, h₃≦h₁.

Description

FIELD OF THE INVENTION

The invention relates to a heat transfer tube having a structured inner surface for evaporation of liquids or condensation of gases, consisting of pure substances or mixtures, on the inside of the tube.

BACKGROUND OF THE INVENTION

Worldwide competition regarding heat exchangers, for example fin-and-tube heat exchangers (compare FIG. 1) for air-conditioning and refrigeration, demands high-performance heat transfer tubes produced with little material and inexpensively in only a few operating steps. The heat transfer tubes are thereby arranged mostly horizontally in fin-and-tube heat exchangers.

Since climate-control devices are often designed to be able to switch over between summer (cooling) and winter (heating) operation, the fin-and-tube heat exchanger and thus the heat transfer tubes of the inside or outside unit of a climate-control system must be operated, depending on the type of operation, sometimes in the evaporation and sometimes in the condensation mode. Accordingly, tubes with good performance characteristics in both modes are often demanded.

State of the Art:

Part of the state of the art is a heat transfer tube according to EP 0 591 094 A1, where the ribs are of the same shape and extend helically at a helix angle with respect to the longitudinal direction of the tube on the inner surface. In particular, during evaporation it is possible for the helical structure to support a complete wetting of the entire tube circumference and it thus achieves an improvement of the heat transfer. However, the heat transfer performance in particular during condensation falls clearly behind when compared to the following discussed structures.

DE 196 28 280 C2 discloses, in circumferential direction of the tube, an alternating change in rib direction taking place in sections. A helical flow cannot develop here because of the lacking unidirectional orientation of the structure in contrast to the helix-shaped structures. This type of structuring of the inner surface is little suited for evaporation since clearly lower evaporation performances are achieved than in tubes where the surface has a clear predominant direction for flow near the wall. On the other hand, this structure shows during condensation excellent heat transfer performances at, however, also a clearly increased pressure drop, because a complete wetting of the surface is not supported by the structure and thus the film thickness in the upper half of the tube limiting the heat transfer during condensation is kept thin.

U.S. Pat. No. 6,229,909 B1 is similar to the DE 196 28 280 C2 in that in the circumferential direction of the tube, an alternating change in rib direction takes place in sections. In order to counter the clearly higher pressure drop of these structures, the rib height was reduced in the transition area between two sections by suitably designing the tool with the disadvantage that the wall thickness increases in this transition area, and the weight of the tube is thus increased without utilizing this additional material neither for improving the heat transfer performance nor for improving the mechanical characteristics. Just as earlier stated, this structure shows very good condensation results, however, compared to the state of the art, clearly inferior evaporation results.

EP 1 087 198 A1 and JP-OS 10-047880 (Kobe Steel) are similar to the DE 196 28 280 C2 in that in the circumferential direction of the tube, an alternating change in rib direction takes place in sections. However, the zones are here designed alternately with different widths so that again a dominating helical-shaped predominant direction of flow can be created, which during evaporation supports the complete wetting of the circumference of the tube and promotes the heat transfer. On the other hand, the helical structure is sufficiently often interrupted so that this structure shows, regarding the condensation performance, similarly good values like the structures according to the DE 196 28 280 C2. Disadvantageous, however, and similar to the DE 196 28 280 C2, is an occurrence of a high pressure drop in the tubes.

In JP-OS 04-158193 (Furukawa), where the inner surface of the tube is divided into sections in circumferential direction of the tube, the rib geometry changes in sections with respect to the helix angle, number of ribs and height of ribs.

In JP-OS 2000-283680 (Kobe Steel), wherein circumferential direction of the tube a change takes place in sections between zones with ribs extending inclined with respect to the longitudinal axis and zones wherein these ribs are additionally notched. Disadvantageous is that the notching of the ribs requires a second rolling step and an additional tool, and thus increases the production cost. In addition, the weight of the tube is not reduced in spite of the forming of the notches since the material is merely displaced into the earlier formed troughs between the ribs.

JP-OS 02-280933 (Furukawa) features a lattice-like rib structure on the entire circumference of the tube. Indeed the secondary ribs are located in the troughs between the primary ribs and hinder the creation of a helical flow and thus a complete wetting of the circumference of the tube, which wetting promotes evaporation, since areas with a clear zone not disturbed by secondary ribs does not exist.

Purpose:

The purpose of the invention is to provide a heat transfer tube having a structured inner surface which optimizes the following demands: a heat transfer performance which is good or improved compared to the state of the art, during both condensation and evaporation, a low pressure drop, a tube weight which is as low as possible, and a reduced production cost calculated according to the number of structure embossing steps.

SUMMARY OF THE INVENTION

The purpose is attained inventively in heat transfer tubes by dividing in circumferential direction the inner surface of the tubes into at least two zones (Z ₁, Z₂, . . . Z_n) extending parallel to the longitudinal axis of the tube, whereby the zones can be differentiated into at least two zone classes (K₁, K₂, . . . , K_m), and zones of a different zone class alternate in circumferential direction at any desired sequence, whereby in zones of at least one zone class (K₁, K₂, . . . , K_j) there extend ribs with a rib height h₁and at a helix angle α₁with respect to the longitudinal direction of the tube so that, when several zone classes (K₁, K₂, . . . , K_j) exist, they differ in at least one of the characteristics of rib height and helix angle, and wherein in zones of at least one further zone class (K_j+1, K_j+2, . . . , K_m) there exist ribs with a helix angle α₂with respect to the longitudinal direction of the tube and with a rib height h₂, which ribs are intersected by ribs having a rib height h₃and extend at a helix angle α₃with respect to the longitudinal direction of the tube (α₃≠α₂), whereby the rib heights h₂and h₃of the intersecting ribs in the zones of the zone classes (K_j+1, K_j+2, . . . , K_m) are the same or preferably smaller than the rib heights h₁of the ribs in the zones of the zone classes (K₁, K₂, . . . , K_j), which zones are the closest in circumferential direction.

From this the following advantages of the invention results:

Through the change of zones with ribs extending at an angle with respect to the longitudinal direction of the tube on the one hand and zones with ribs intersecting in a lattice-like pattern on the other hand, there exists through the first-mentioned zones the possibility to construct a preferred direction for a helical flow, which due to its helix configuration supports a complete wetting of the circumference of the tube, and thus contributes to a good and improved heat transfer performance during evaporation. On the other hand, this helical flow is in each case briefly broken down by the, preferably, however, not necessarily, narrower zones having a lattice-like pattern which assures a creation of turbulence and a destruction of temperature and concentration boundary layers and can thus further increase the heat transfer prior to the flow being again reestablished in the preferred helical direction. The intersection angle of the intersecting ribs in the zones of the zone classes (K _j+1, K_j+2, . . . , K_m) having a lattice-like pattern, calculated as the amount of the smaller of the two complementary angles |(α₂-α₃)| or |180°−(α₂-α₃)|, is preferably 30° to 90°.

For the condensation operation on the other hand, the helical structure exhibiting a predominant direction characteristic is sufficiently often interrupted and a helical flow is disturbed, by the zones of the zone classes (K _j+1, K_j+2, . . . , K_m) having ribs intersecting in a lattice-like pattern so that, in the upper half of the tube, a removal of the condensate occurs which results in a reduction of the film thickness of the condensate. This structure shows therefore a very good condensation performance. The chosen width of the zones of a lattice-like pattern of the zone classes (K_j+1, K_j+2, . . . , K_m) and the thus forced breakdown of a pure helical flow represents a compromise between good evaporation and condensation performances. The width of the zones of intersecting ribs is preferably chosen to be narrower than the zones with a straight ribbing; in particular, the width of the zones of intersecting ribs should be 3-70% of the width of the zones having straight ribbing.

The inventive structure has compared to the state of the art in EP 1 087 198 a reduced pressure drop resulting from the reduction of the height of the ribs in the zones of a lattice-like pattern of the zone classes (K_j+1, K_j+2, . . . , K_m), which zones extend parallel with respect to the longitudinal direction of the tube. The ribs are in comparison to the ribs of the height h₁designed with a preferably lower rib height h₂or h₃. Thus, the flow following the helix encounters, in contrast to the state of the art according to EP 1 087 198, merely elevations of a lesser height.

Of course the material available through the reduction of the height of the elevations does not cause, as this is the case in U.S. Pat. No. 6,298,909, an unnecessary local reinforcement of the wall thickness and thus in an unnecessary increase in the weight of the tube, but is instead utilized according to the invention for the construction of the lattice-like pattern or the intersecting ribs for the further enhancement of the heat transfer surface and lastly the performance, whereas the wall thickness is uniform in circumferential direction of the tube when measured at the base of the grooves between the ribs of the height h ₁in the zones of the zone classes (K₁, K₂, . . . , K_j) or in the cavities between the ribs in the zones of the lattice-like pattern of the zone classes (K_j+1, K_j+2, . . . , K_m) outside of a possibly existing welding-seam section. It can furthermore be assured in this manner that in spite of the reduced rib height in the zones of the lattice-like pattern of the zone classes (K_j+1, K_j+2, . . . , K_m) there exist with respect to the strip elongation during rolling comparable values like in the zones of the zone classes (K₁, K₂, . . . , K_j). Unnecessary stress and a possibly occurring waviness of the strip can thus be avoided.

A further advantage of the inventive structure is that this structuring can be achieved in one single rolling step and with one single rolling tool. Compared with notched structures, the production expense calculated according to the number of rolling and operating steps is in this manner reduced. However, an additional notching of the ribs in individual zones of the zone classes (K ₁, K₂, . . . , K_j) can show further advantages, in particular with regard to a further increase in performance.

The inventive heat transfer tube is manufactured based, for example, on the method described in greater detail hereinafter. Usually copper or a copper alloy is used as the material for the heat transfer tube, however, the present invention is not limited in this manner. Rather any type of metal can be used, for example, aluminum. Initially a metallic flat strip is subjected to a one-step embossing step by being guided between an emboss roll having a surface design complementary to the inventive structure and a support roller. One side of the flat strip receives thereby the inventive structure, whereas the second side remains smooth or has also a structuring here not described in detail. Merely the edge areas of the first side, which edge areas are used for the subsequent welding, may possibly be structured otherwise or may also remain non-structured. Following the embossing step, the structured flat strip is formed into an open seam tube, is longitudinally seam welded during a welding process, and the tube is, if necessary, during a subsequent drawing process brought to the desired outside diameter. A possible influence on the heat transfer ability of the inventive heat transfer tube through the area surrounding the welding seam, otherwise structured or also non-structured is insignificant and can be neglected.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be discussed in greater detail in connection with the following exemplary embodiments illustrated in the drawings, in which: [0022]
FIG. 1 illustrates a fin-and-tube heat exchanger according to the state of the art; [0023]
FIG. 2 isometrically illustrates a fragment of an internally ribbed heat transfer tube, in which a welding-seam section extends in a longitudinal direction of the tube; [0024]
FIG. 3 isometrically illustrates a top view of a flattened inventive heat transfer tube having a ribbed inner surface; [0025]
FIG. 4 isometrically illustrates the definition of the helix angle α; [0026]
FIG. 5 isometrically illustrates a top view of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 3, in which in the zones of even numbers the intersecting ribs form a lattice-like pattern; [0027]
FIG. 6 isometrically illustrates a top view of a further embodiment of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 3; [0028]
FIG. 7 isometrically illustrates a top view of a further embodiment of a flattened inventive heat transfer tube having a ribbed inner surface, in which in the zones of uneven numbers the helix angle differs from zone to zone; [0029]
FIG. 8 isometrically illustrates a top view of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 7, in which the width of the zones differs; [0030]
FIG. 9 isometrically illustrates a top view of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 5, in which the ribs are notched in the zones of uneven numbers (Z[0031] ₁, Z₃, . . . );
FIG. 10 illustrates in an enlarged scale a cross section taken along the line A-A in FIG. 9; and [0032]
FIG. 11 schematically illustrates the design of an embossing roller for the manufacture of the inventive heat transfer tubes.[0033]

DETAILED DESCRIPTION

FIG. 1 illustrates a fin-and-tube heat exchanger according to the state of the art with horizontally arranged [0034] heat transfer tubes 4 and fins not identified in detail.
FIG. 2 illustrates a longitudinal section of a longitudinally seam-welded [0035] heat transfer tube 4 having an outside diameter D. The heat transfer tube 4 has a plain outer surface, a structured inner surface and a welding-seam section 7. The performance of an inventive heat transfer tube 4 is not significantly influenced by the slight interruption of the structure of the inner surface at the welding-seam section 7 and is negligible. The welding-seam section 7 extends parallel to the longitudinal axis of the tube and lies between two zones Z, which will be illustrated in greater detail in the following figures, without noticeably influencing the effect of the zone change.
FIG. 3 illustrates schematically a top view of the flattened inner surface of an inventive [0036] heat transfer tube 4. The inner surface is divided in circumferential direction into five zones (Z₁to Z₅) of different width (B₁to B₅), whereby in zones (Z₁, Z₃, . . . ) of the zone class K₁ribs 1 extend at a helix angle α₁with respect to the longitudinal direction of the tube. Ribs 2 in the zones (Z₂, Z₄, . . . ) of the zone class K₂extend at a helix angle α₂and at the same rib height h₂, the ribs 2 being intersected by ribs 3 of the same height. The associated zones have within one zone class K the same structuring with respect to the rib pattern, the rib height and the helix angle. The respective helix angle α₂and α₃differ from one another at the intersecting ribs 2 and 3 in the zones of the zone class K₂. Also illustrated is the core-wall thickness t. The ribs 2 are in this particular embodiment arranged in alignment with the ribs 1 and extend at the same helix angle (α₂=α₁) with respect to the longitudinal direction of the tube. The widths of the zones of one zone class are in each case the same, whereas the zones (Z₁, Z₃, Z₅) of the zone class K₁are designed wider than the zones (Z₂, Z₄) of the zone class K₂.
FIG. 4 schematically illustrates the definition of the helix angles α. The longitudinal direction of the tube is thereby identified as the zero degree point (0°), whereas the [0037] ribs 1 a, which extend away from the 0° line to the right in longitudinal direction of the tube, are described as a positive angle (α>0), and ribs 1 b, which extend away from the 0° line to the left in longitudinal direction of the tube, are described as a negative angle (α<0).
FIG. 5 schematically illustrates the top view of a flattened, inventive heat transfer tube having a ribbed inner surface analogous to FIG. 3, in which in the zones of the zone class K[0038] ₂the intersecting ribs 2 and 3 form an intersection angle, calculated as the amount of the smaller one of the two complementary angles |(α₂-α₃)| or |(180°−(α₂-α₃))|, of approximately 40°. The intersecting ribs 2 and 3 completely enclose thereby a cavity 5 in the zones of the zone class K₂by forming a closed diamond-shaped rib pattern 6. A lattice-like pattern is thus created. The ribs 2 and 3 in the zones of the zone class K₂are thereby designed to have a rib height of h₂or h₃smaller than the height h₁of the ribs 1 in the zones of the zone class K₁. The ribs 3 extend at an angle α₃with respect to the longitudinal direction of the tube. The core-wall thickness t of the heat transfer tube 4, measured at the base of the groove 9 between the ribs 1 in the zones (Z₁, Z₃, . . . ) of the zone class K₁or in the cavities 5 between the ribs 2, 3 in the zones (Z₂, Z₄, . . . ) of the zone class K₂outside of a welding-seam section 7, is uniform in circumferential direction of the tube.
FIG. 6 schematically illustrates a top view of a further embodiment of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 3, in which in the zones, (Z[0039] ₂, Z₄, . . . ) of the zone class K₂the intersecting ribs 2 and 3 form an intersection angle, calculated as the amount of the smaller one of the two complementary angles |(α₂-α₃)| or |180°−(α₂-α₃)|, of approximately 90°.
FIG. 7 illustrates schematically a top view of a further embodiment of a flattened inventive heat transfer tube having a ribbed inner surface, in which the zones Z[0040] ₁to Z₅are divided into three zone classes K₁to K₃. The helix angle of the ribs 1 is α₁in the zones (Z₁, Z₅) of the zone class K₁, whereas the helix angle is α₁* in the zone Z₃of the zone class K₂. The helix angle of the ribs 1 with respect to the longitudinal direction of the tube is, in the illustrated embodiment, alternately changed in the zones of uneven number (Z₁, Z₃, Z₅) from zone to zone between α₁and α₁*. The intersecting ribs 2 and 3 form in the zones (Z₂, Z₄) of the zone class K₃a lattice-like pattern by completely enclosing several cavities 5 each in a closed diamond-shaped rib pattern 6.
FIG. 8 illustrates schematically a top view of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 7, in which the width of the zones of the zone class K[0041] ₃(B₂, B₄) is only approximately 50% of the width of the zones of the zone classes K₁and K₂(B₁, B₃, B₅).
FIG. 9 schematically illustrates a top view of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 5, in which the ribs have [0042] notches 8 in individual zones. That is, the ribs 1 of the zone Z₃of the zone class K₂have notches 8 in the illustrated embodiment, which notches lie in alignment one behind the other on lines which extend at a helix angle α₄with respect to the longitudinal direction of the tube. The depth k of the notches 8 according to the illustration in FIG. 10 is at least 20% of the rib height h₁of the ribs 1.
FIG. 11 schematically illustrates the design of an [0043] embossing roller 11 for the manufacture of the inventive heat transfer tube 4. The roller 11 is composed of several circular discs 12. Grooves 13, 14, 15 are recessed into the peripheral surface of the individual discs 12, which grooves produce, when the roller 11 rolls on the sheet metal strip 10 supported by a smooth surface support roller 16, the ribs 1, 2, 3 in the individual zones Z₁to Z₅during one rolling operation. According to the illustrated structuring, the sheet metal strip 10 is formed into an open seam tube and is welded along a longitudinal seam so that a welding-seam section 7 results.
Numerical Example: [0044]
One embodiment of a flattened inventive heat transfer tube having a ribbed inner surface analogous to FIG. 5 is featured by an outer diameter of the tube being 9.52 mm and an inner surface which is divided in the circumferential direction of the tube into seven zones of different width. The width of the zones is alternately specified by a circumferential angle of 72° (4 wide zones) or 24° (3 narrow zones). Twelve [0045] ribs 1 each with a rib height h₁of 0.25 mm are formed in the circumferential direction in the wide zones, which ribs extend at a helix angle α₁of +20° with respect to the longitudinal axis of the tube, and are continued in the narrow zones in alignment at the same helix angle (α₂=α₁), however, with a reduced rib height h₂of 0.15 mm. Thus, four ribs 2 exist in circumferential direction in the narrow zones. The ribs 2 are intersected by ribs 3 in the zones of even numbers, which ribs 3 extend at an opposite helix angle α₃of −20° with respect to the longitudinal direction of the tube, so that the intersection angle between the ribs 2 and 3 is 40°. The rib height h₃is 0.15 mm. The density of the ribs 3 in the zones of even numbers (Z₂, Z₄, . . . ), measured as number of ribs per unit of length in direction of the ribs 2, is 1.45 per millimeter. This embodiment of an inventive heat transfer tube showed, compared with a tube according to the state of the art, particularly good characteristics regarding the heat transfer performance and the pressure drop while the tube has a low weight per meter of the tube.

Claims

What is claimed is:

1. In a heat transfer tube (4), the inner surface of which is divided in circumferential direction into at least two zones (Z₁, Z₂, . . . , Z_n) extending parallel to the longitudinal axis of the tube, and

wherein the zones are classified into at least two zone classes (K₁, K₂, . . . , K_j, K_j+1, . . . , K_m), and

wherein zones of a different zone class alternate at any desired sequence in circumferential direction,

whereby in zones of at least one zone class (K₁, K₂, . . . , K_j) there extend ribs (1) having a rib height h₁and at a helix angle α₁with respect to the longitudinal direction of the tube so that, when several of these zone classes (K₁, K₂, . . . , K_j) exist, they differ in at least one of the characteristics of rib height and helix angle of inclination,

the improvement wherein:

in zones of at least one further zone class (K_j+1, K_j+2, . . . , K_m) there exist ribs (2) having a helix angle α₂with respect to the longitudinal direction of the tube and a rib height h₂, and which ribs (2) are intersected by ribs (3) having a rib height h₃, which ribs (3) extend at a helix angle α₃with respect to the longitudinal direction of the tube (α₃≠α₂), whereby the rib heights h₂and h₃of the intersecting ribs (2) and (3) in the zones of the zone classes (K_j+1, K_j+2, . . . , K_m) are the same or preferably smaller than the rib heights h₁of the ribs (1) in the zones of the zone classes (K₁, K₂, . . . , K_j), which zones are the closest in circumferential direction.

2. The heat transfer tube according to claim 1, wherein the interesting ribs (2) and (3) form an intersection angle of 30° to 90° in the zones of the zone classes (K_j+1, K_j+2, . . . , K_m).

3. The heat transfer tube according to claim 1, wherein the rib density of the ribs (3), measured as number of ribs per unit of length in direction of the ribs (2), is 0.5-4 per millimeter, preferably 1-3 per millimeter, in the zones of the zone classes (K_j+1, K_j+2, . . . , K_m).

4. The heat transfer tube according to claim 1, wherein the intersecting ribs (2) and (3) produce a lattice-like pattern in the zones of the zone classes (K_j+1, K_j+2, . . . , K_m), in which pattern the ribs (2) and (3) of one zone enclose at least one cavity (5) in one closed rib pattern.

5. The heat transfer tube according to claim 1, wherein the zones are classified into two zone classes (K₁, K₂), and the zones of the zone classes K₁and K₂alternate in circumferential direction, whereby ribs (1) with a rib height h₁and a helix angle α₁extend in longitudinal direction of the tube in zones of the zone class K₁, and ribs (2) extend in alignment with the ribs (1) at the same helix angle of inclination α₂(α₂=α₁) with respect to the longitudinal direction of the tube with the rib height h₂in zones of the zone class K₂, and are intersected by ribs (3) having a rib height h₃, which ribs (3) extend at a helix angle α₃with respect to the longitudinal direction of the tube (α₃≠α₂).

6. The heat transfer tube according to claim 5, wherein the width B of the zones of one zone class, measured in circumferential direction, is the same in each case, and the width of the zones of the zone class K₂is less than the width of the zones of the zone class K₁.

7. The heat transfer tube according to claim 6, wherein the width of the zones of the zone class K₂, measured in circumferential direction, is 3% to 70% of the width of the zones of the zone class K₁.

8. The heat transfer tube according to claim 5, wherein the rib height h₁of the ribs (1) is 0.15-0.40 mm.

9. The heat transfer tube according to claim 1, wherein the core-wall thickness of the heat transfer tube, measured at the base of a groove (9) between the ribs (1) in the zones of the zone classes (K₁, K₂, . . . , K_j) or in the cavities (5) between the ribs (2, 3) in the zones of the zone classes (K_j+1, K_j+2, . . . , K_m) outside of a welding-seam section (7), is uniform in circumferential direction of the tube.

10. The heat transfer tube according to claim 1, wherein the ribs (1) have notches (8) in individual or several zones of the zone classes (K₁, K₂, . . . , K_j) whereby the notches (8) extend aligned at an angle α₄, which is different from the helix angle α₁of the ribs (1) of the respective zone, with respect to the longitudinal direction of the tube, and the notch depth k is at least 20% of the rib height h₁of the ribs (1) of the respective zone.